Wet chemical noble metal coating

ABSTRACT

The invention relates to a method for coating a substrate with a noble metal layer, which comprises the following steps: (i) providing a substrate; (ii) applying a liquid noble metal ink to the substrate, wherein the noble metal ink contains less than 10 percent by weight of noble metal, based on the total weight of the noble metal ink; and (iii) heating the liquid noble metal ink, and thereby forming a noble metal layer on the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority pursuant to 35 U.S.C. 119(a) to German Application No. 102022111991.2, filed May 12, 2022, which application is incorporated herein by reference in its entirety.

TECHNICAL BACKGROUND

Owing to their good biological compatibility, their good electrical conductivity and the high corrosion resistance, noble metals such as gold and platinum are used in many ways in medical technology, for example, for electrochemical sensors, surgical instruments and implantable medical devices. Due to the high costs of these noble metals, in many cases, only a thin coating is applied to a substrate. However, it is frequently a challenge to apply a layer of noble metal to the substrate that is as thin as possible but at the same time homogeneous and closed. Many methods suitable for applying thin layers of a noble metal are expensive and complex. With other methods, it is often not possible to apply thin layers. For example, wire electrodes having a diameter of approximately 100 µm are used in medical electrochemical sensors. In the case of sheath wires having an outer layer made of, for example, platinum, this outer layer typically amounts to approximately 1 to 20 µm, and therefore the noble metal content of these wires is very high, which leads to correspondingly high costs.

DE102019219615A1 describes production methods for medical electrodes, in which a dissolved organic noble metal complex compound is used for applying a noble metal layer.

PREFERRED EMBODIMENTS

The object of the present invention is to solve one or more of the problems described above and further problems of the prior art. For example, the invention enables the production of surgical instruments or implantable medical devices which comprise a particularly thin, smooth and microscopically closed noble metal layer, for example, a platinum layer. Furthermore, the present invention provides compositions which can be used for the corresponding production methods.

These objects are achieved by the methods and devices described herein, in particular those that are described in the claims.

Preferred embodiments of the invention are described below.

|1| A method for coating a substrate with a noble metal layer, comprising the following steps:

-   (i) providing a substrate, -   (ii) applying a liquid noble metal ink to the substrate, wherein the     noble metal ink contains less than 10 percent by weight of noble     metal, preferably less than 9, 8, 7, 6 or less than 5 percent by     weight of noble metal, based on the total weight of the noble metal     ink, -   (iii) heating the liquid noble metal ink and thereby forming a noble     metal layer on the substrate.

|2| The method according to embodiment 1, wherein in step (iii) a noble metal layer is formed which has a layer thickness of less than 100 nm, preferably less than 50 nm, 40 nm, 35 nm, 30 nm or less than 25 nm.

131 The method according to either of the preceding embodiments, wherein in step (iii) a noble metal layer is formed which is completely closed.

141 The method according to one of the preceding embodiments, wherein in step (iii) a noble metal layer is formed which has a roughness Ra of less than 100 nm.

|5| The method according to one of the preceding embodiments, wherein the noble metal ink is particle-free.

|6| The method according to one of the preceding embodiments, wherein in step (iii) 50 to 500 µg, preferably 50 to 200 µg, of noble metal per cm² of substrate surface are applied to the substrate.

|7| The method according to one of the preceding embodiments, wherein the noble metal layer obtained in step (iii) makes up less than 10 percent by volume, preferably less than 5 or less than 1 percent by volume, of the entire substrate-noble metal layer composite.

|8| The method according to one of the preceding embodiments, wherein the liquid noble metal ink is applied by means of inkjet printing, screen printing, stamp printing, application by means of felt, dispensing, dip coating, spray coating, die coating or spin coating.

|9| The method according to one of the preceding embodiments, wherein the substrate provided with the noble metal layer is suitable for implantation in the animal or human body.

|10| The method according to one of the preceding embodiments, wherein the substrate provided with the noble metal layer is a belt, a cylinder, a tube, a sphere, a wire, or preferably a catheter, a stent, an electrode, a sensor or a stylet.

|11| The method according to one of the preceding embodiments, wherein the substrate provided with the noble metal layer comprises a noble metal, a base metal or a non-metal.

|12| The method according to one of the preceding embodiments, wherein the noble metal ink comprises platinum, gold or silver.

|13| The method according to one of the preceding embodiments, wherein the noble metal ink comprises an organic noble metal complex having diolefin and C6-C18 monocarboxylate ligands of the type [(L1L2)Pt[O(CO)R¹]₂]_(n), [LPd[O(CO)R1]X]_(n), [LRh[O(CO)R1]]_(m) or [LIr[O(CO)R1]]_(m), wherein L denotes a compound acting as a diolefin ligand, wherein X is selected from bromide, chloride, iodide and -O(CO)R2, wherein -O(CO)R1 and -O(CO)R2 denote identical or different non-aromatic C6-C18 monocarboxylic acid groups, and wherein n is a whole number ≥ 1 and m is a whole number ≥ 2.

|14| The method according to embodiment 13, wherein the organic noble metal complex is a compound with the formula [(L1L2)Pt[O(CO)R¹]₂]_(n), wherein n is equal to 1 or 2, L1L2 is cyclooctadiene or norbornadiene, and wherein R¹ represents a non-aromatic C7-C17 hydrocarbon group.

|15| The method according to either of embodiments 13 or 14, wherein the noble metal complex is a compound with the formula (COD)Pt[O(CO)C(CH₃)₂C₆H₁₃]₂.

|16| The method according to one of embodiments 12 to 15, wherein the noble metal ink comprises a solvent which preferably comprises propylene glycol n-propyl ether and/or ethanol.

|17| A substrate having a noble metal layer, which can be produced with the aid of a method according to one of the preceding embodiments.

|18| A noble metal ink, comprising less than 10 percent by weight of noble metal, preferably less than 9, 8, 7, 6 or less than 5 percent by weight of noble metal, based on the total weight of the noble metal ink, and a solvent, wherein the noble metal is present as an organic noble metal complex dissolved in the solvent.

|19| The noble metal ink according to embodiment 18, wherein the noble metal complex comprises diolefin and C6-C18 monocarboxylate ligands of the type [(L1L2)Pt[O(CO)R¹]₂]_(n), [LPd[O(CO)R1]X]_(n), [LRh[O(CO)R1]]_(m) or [LIr[O(CO)R1]]_(m), wherein L denotes a compound acting as a diolefin ligand, wherein X is selected from bromide, chloride, iodide and -O(CO)R2, wherein -O(CO)R1 and -O(CO)R2 denote identical or different non-aromatic C6-C18 monocarboxylic acid groups, and wherein n is a whole number ≥ 1 and m is a whole number ≥ 2.

|20| The noble metal ink according to embodiment 18 or 19, wherein the noble metal complex is a compound with the formula [(L1L2)Pt[O(CO)R¹]₂]_(n), wherein n is equal to 1 or 2, L1L2 is cyclooctadiene or norbornadiene, and wherein R¹ represents a non-aromatic C7-C17 hydrocarbon group.

|21| The noble metal ink according to one of embodiments 18 to 20, wherein the noble metal complex is a compound with the formula (COD)Pt[O(CO)C(CH₃)₂C₆H₁₃]₂.

|22| A material composite, comprising a substrate and a completely closed noble metal layer, which has a layer thickness of less than 100 nm, preferably less than 50 nm or less than 30 nm.

|23| An implantable device for medical use, comprising a composite material according to embodiment 22.

DETAILED DESCRIPTION

In principle, for the embodiments described herein, the elements of which “contain” or “comprise” a particular feature (e.g., a material), a further embodiment is always considered in which the element in question consists of that feature alone, i.e., comprises no further components. The word “comprise” or “comprising” is used herein synonymously with the word “contain” or “containing.”

If an element is referred to in the singular in an embodiment, an embodiment is also considered in which a plurality of these elements are present. The use of a term for an element in the plural fundamentally also encompasses an embodiment in which only a single corresponding element is contained.

Unless otherwise indicated or clearly precluded from the context, it is possible in principle, and is herewith clearly taken into consideration, that features of different embodiments may also be present in the other embodiments described herein. It is also contemplated in principle that all features that are described herein in conjunction with a method are also applicable to the products and devices described herein, and vice versa. Only for reasons of succinct presentation are all such contemplated combinations not explicitly listed in all instances. Technical solutions which are known to be equivalent to the features described herein are also intended to be encompassed in principle by the scope of the invention.

The invention relates to a method for coating a substrate with a noble metal layer, comprising the following steps;

-   (i) providing a substrate; -   (ii) applying a liquid noble metal ink to the substrate, wherein the     noble metal ink contains less than 10 percent by weight of noble     metal, preferably less than 9, 8, 7, 6 or less than 5 percent by     weight of noble metal, based on the total weight of the noble metal     ink; and -   (iii) heating the liquid noble metal ink and thereby forming a noble     metal layer on the substrate.

The noble metal content of the noble metal ink is always specified herein based on the pure metal content (e.g., the platinum content of a platinum complex compound) in relation to the total weight of the noble metal ink.

The substrate may comprise a noble metal, a base metal, or a non-metal. Examples of noble metals according to the invention are platinum, iridium, palladium, gold, silver, ruthenium and rhodium.

The substrate can comprise a metal, preferably a biocompatible metal. Suitable biocompatible metals are known in the art, for example Pt, Ir, Ta, Pd, Ti, Fe, Au, Mo, Nb, W, Ni, Ti, or a mixture or alloy thereof. Whether a metal is biocompatible can be determined using the standard EN ISO 10993.

In some embodiments, the substrate comprises or consists of the alloy MP35N, MP35NLT, PtIr10, Ptlr20, 316L, 301, 304 or nitinol. The substrate can also comprise multilayer material systems. In some embodiments, the substrate consists of one or more of these materials.

MP35N is a nickel-cobalt-based hardenable alloy. A variant of MP35N is described in industry standard ASTM F562-13. In one embodiment, MP35N is an alloy comprising 33 to 37 % Co, 19 to 21 % Cr, 9 to 11 % Mo, and 33 to 37 % Ni.

MP35NLT is an alloy that contains the following alloying components:

-   Cr in the range of from 10 to 30 wt.%; -   Ni in the range of from 20 to 50 wt.%; -   Mo in the range of from 2 to 20 wt.%; -   Co in the range of from 10 to 50 wt.%; -   wherein the Al content of the alloy is less than 0.005 wt.%; -   wherein preferably at least 99.95 wt.% of the alloy consists of Cr,     Ni, Mo and Co; -   wherein each weight percent is based on the total weight of the     alloy.

Some preferred variants of MP35NLT are described in EP3181710A1.

PtIr10 is an alloy of 88 to 92 % platinum and 8 to 12 % iridium.

PtIr20 is an alloy of 78 to 82 % platinum and 18 to 22 % iridium.

316L is an acid-resistant CrNiMo austenitic steel with approx. 17 % Cr; approx. 12 % Ni, and at least 2.0 % Mo. A variant of 316L is described in industry standard 10088-2. In one embodiment, 316L is an alloy comprising 16.5 to 18.5 % Cr; 2 to 2.5 % Mo, and 10 to 13 % Ni.

301 is a chromium-nickel steel with high corrosion resistance. A variant of 301 is described in industry standard DIN 1.4310. In one embodiment, 301 is an alloy comprising 16 to 18 % Cr and 6 to 8 % Ni.

304 is an austenitic, acid-resistant 18/10 Cr-Ni steel, which is described, for example, in manufacturing standards ASTM A213, ASTM A269, ASTM A312 or ASTM A632. 304 typically contains 8-10.5 % nickel, 18-20 % chromium, up to 2 % manganese and up to 0.08 % carbon. A variant of 304 is 304L, which contains up to 12 wt.% of nickel.

Nitinol is a shape-memory nickel-titanium alloy having an ordered cubic crystal structure and a nickel content of approximately 55 %, the remaining portion consisting of titanium. Nitinol has good biocompatibility and corrosion-resistance properties.

Unless otherwise indicated, all percentages given herein are to be understood as weight percent (wt.%).

Alternatively, or additionally, the substrate can comprise a non-metal.

A non-metal is any material that is not a metal. The non-metal can be, for example, a polymer, glass or carbon. The substrate may be present as a completely filled body, or in the form of a porous structure and/or fibrous structure. For example, the substrate may comprise glass fibers or carbon fibers.

Preferred polymers are biocompatible polymers. Examples of biocompatible polymers include polyimide, polyethylene, polyurethane, PEEK, polyoxymethylene (e.g., Delrin®), LCP (liquid crystal polymers), and silicone. In one embodiment, the substrate comprises polyimide, for example Kapton.

The substrate can in principle have any desired geometry. The substrate may be, for example, a tape, a cylinder, a tube, a sphere, or a wire, or may comprise such a form or combinations thereof. For example, it can be a metal wire, a metal foil, a polymer film or a surgical instrument made of metal and/or plastics material. The substrate can also comprise mesh structures, partially open structures or complex wire geometries, as occur, for example, in medical stents. The material of the substrate should preferably be sufficiently heat-resistant to withstand the curing of the noble metal ink without deformation, change in material or destruction. What is crucial here is the decomposition temperature of the noble metal ink used. Some of the noble metal complexes described herein have particularly low decomposition temperatures, and therefore comparatively temperature-sensitive substrates can also be coated.

A liquid noble metal ink is applied to the substrate. For this purpose, any suitable method can be used, for example inkjet printing, screen printing, stamp printing, dispensing, dip coating, die coating (slot die coating), spray coating or spin coating. Examples thereof are disclosed in DE102019219615A1.

Preferably, the noble metal ink is applied as homogeneously as possible to the substrate in order to later obtain a correspondingly homogeneous noble metal layer.

It may be advisable to meter the amount of noble metal in such a way that a layer thickness that is both completely closed and also as thin as possible is achieved. For this purpose, it may be helpful, in addition to the use described above of a low noble metal concentration of less than 10 percent by volume, to apply a correspondingly small amount of the noble metal ink to the substrate. In some embodiments, for example, 50 to 500 µg, preferably 50 to 200 µg, of noble metal per cm² of substrate surface in the form of said noble metal ink are therefore applied to the substrate.

In some embodiments, the resulting noble metal layer has a layer thickness of less than 100 nm, preferably less than 50 nm or less than 30 nm.

In some embodiments, the noble metal layer obtained makes up less than 10 percent by volume, preferably less than 5 or less than 1 percent by volume, of the entire substrate-noble metal layer composite, i.e., less than 10, 5 or 1 percent by volume of the composite of substrate and noble metal layer consists of a noble metal applied according to the invention.

With the aid of the methods described herein, thin noble metal layers which form a closed layer can be produced. This means that the noble metal layers formed are substantially free of defects, even on a microscopic scale. In particular, the noble metal layers produced according to the invention preferably contain virtually no cracks on the micrometer and three-digit nanometer scale, i.e., no cracks with a width of more than 100 nm are observed on the surface of the noble metal layer according to the invention. As a result, the layers according to the invention can be used, for example, effectively as corrosion protection for medical instruments or as an active outer coating for electrochemical measuring electrodes. Whether such a closed layer according to the invention is present can be determined, for example, by means of a nickel spot test using dimethylglyoxime or by means of a ferroxyl test (e.g., by means of commercial tests such as Henkel Ferroxyl Test HC 7000), or with the aid of a suitable electrochemical detection method, for example cyclic voltammetry, wherein the absence of oxidation peaks of a substrate comprising a non-noble metal is indicative of a closed layer according to the invention (see example 11 herein). Depending on the type of substrate used, a correspondingly suitable method can be used. The decisive factor is that no significant electrochemical activity or accessibility of the substrate is detectable.

The noble metal layers described herein may have particularly low roughness. For example, a noble metal layer described herein may have a roughness Ra of less than 100 nm, preferably less than 90, 80, 70, 60, or less than 50 nm.

Particle-free noble metal inks have proven to be particularly advantageous in the context of the methods described herein. This means that the noble metal content in the noble metal ink is completely dissolved, i.e., the noble metal is present in completely dissociated, dissolved form, and not as a suspension. For this purpose, noble metal complex compounds which are soluble in an organic solvent are preferably used. Particle-free noble metal inks are characterized in comparison to particle-containing compositions by better usability in various coating methods, in particular in nozzle-based techniques such as spray coating and printing techniques such as inkjet printing. In addition, such inks are distributed particularly homogeneously on the substrate surface and thereby lead to the formation of particularly homogeneous noble metal layers on the substrate.

The noble metal ink comprises a solvent. Examples of organic solvents according to the invention include aliphates and cycloaliphates, each having 6 to 12 carbon atoms; halocarbons, such as di-, tri-, and tetrachloromethane; aromatics; araliphates, such as toluene or xylene; alcohols, such as ethanol, n-propanol, and isopropanol; ethers; glycol ethers, such as mono-C1-C4 alkyl glycol ethers and di-C1-C4 alkyl glycol ethers, for example ethylene glycol mono-C1-C4 alkyl ether, ethylene glycol di-C1-C4 alkyl ether, diethylene glycol mono-C1-C4 alkyl ether, diethylene glycol di-C1-C4 alkyl ether, propylene glycol mono-C1-C4 alkyl ether, propylene glycol di-C1-C4 alkyl ether, dipropylene glycol mono-C1-C4 alkyl ether, and dipropylene glycol di-C1-C4 alkyl ether; esters having 2 to 12 carbon atoms; and ketones, such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone. Further examples of preferred solvents are propylene glycol n-propyl ether or ethanol.

In certain embodiments, the solvent is a non-aqueous solvent. In some embodiments, the solvent can be used for dip coating or inkjet applications. In some embodiments, the solvent can be used for coating plastics materials, for example, PEEK or polyimide. In some embodiments, the solvent has good wetting properties with respect to plastics materials. In some embodiments, the solvent has good wetting properties with respect to metals, such as stainless steel, nitinol, MP35N, MP35NLT, platinum or platinum iridium alloys. In some embodiments, the solvent has a medium polarity. For example, the dipole moment of the solvent can be 1 to 10; 1 to 8; 1 to 5; or 2 to 4 × 10⁻³⁰ Cm.

In one embodiment, the solvent is a protic solvent. In one embodiment, the solvent is an aprotic solvent.

The noble metal ink according to the invention contains 0 to 10 wt.%, preferably 0 to 3 wt.% of at least one additive. Accordingly, the noble metal ink according to the invention can be additive-free or contain up to 10 wt.% of at least one additive. Examples of additives include wetting additives, rheological additives, defoamers, deaerators, additives for influencing the surface tension, and odorants.

By subsequently heating the liquid noble metal ink, a noble metal layer is formed on the substrate. The heating preferably leads to all other components of the noble metal ink other than the pure noble metal, i.e., for example, the solvent and any organic components of a noble metal complex compound, to completely or almost completely evaporate and/or decompose. A noble metal ink described herein containing a noble metal complex or a corresponding coating comprising noble metal complexes can first be dried and freed of organic solvent before it or the dried residue is subjected to decomposition by thermal treatment to form a noble metal layer. When working with the embodiment of palladium complexes according to the invention, palladium metal is formed as a layer during the thermal decomposition even in the presence of air as ambient atmosphere; in contrast, however, during the thermal decomposition in the embodiment of rhodium or iridium complexes according to the invention, no noble metal layers are formed in the presence of air as ambient atmosphere, but rather the corresponding noble metal oxide layers are formed. In this regard, a person skilled in the art understands the expression “noble metal layer” used herein to be, for example, a layer comprising or consisting of platinum or palladium, a layer comprising or consisting of rhodium oxide, or a layer comprising or consisting of iridium oxide. The thermal treatment comprises heating to an object temperature above the decomposition temperature of a noble metal complex according to the invention or a combination of noble metal complexes according to the invention. For this purpose, for example, the noble metal ink is heated briefly to a temperature above the relevant decomposition temperature range of, for example, 150° C. to 200° C., i.e., for example, correspondingly to > 150° C. to >200° C., for example to 1,000° C., for example in a furnace and/or via infrared irradiation. In some embodiments, an object temperature is selected to be slightly above the decomposition temperature. In some embodiments, the heating, more precisely stated the maintenance of the object temperature, does not require longer than 15 minutes.

The layers comprising noble metal may even be produced on temperature-sensitive substrates, i.e., for example, on substrates that are not temperature-stable above 200° C. For example, these may be temperature-sensitive polymer substrates, for example those based on polyolefin or polyester.

In some embodiments, a plurality of layers of a noble metal are applied in succession, each with the aid of a noble metal ink described herein. For this purpose, a first layer of a noble metal ink can first be applied, and the ink can be dried until substantially no more liquid remains on the substrate. The substrate is now coated with a solid layer of a precursor material, for example, a noble metal complex contained in the ink. Optionally, such a noble metal complex can be thermally decomposed in order to convert the noble metal complex into metallic noble metal. Alternatively, however, a further layer of the noble metal ink can also be applied immediately, and all applied layers of the noble metal complex can be thermally decomposed together after all desired layers of a noble metal complex have been applied to the substrate. In this way, layers of a noble metal having a different thickness can be applied to the substrate, which layers have the properties described herein.

The method according to the invention is particularly useful for noble metal coating of implantable medical devices. In some embodiments, therefore, a method is provided, wherein the substrate provided with the noble metal layer is suitable for implantation in the animal or human body. This means that the noble metal coating according to the invention can be suitable for making a substrate biocompatible, or for producing a biocompatible substrate-noble metal composite.

The substrate may be an implantable medical device or a preliminary product thereof, such as a wire, a catheter, an electrode, a sensor, a stent, or a stylet. A stylet is also referred to as a mandrin. By means of the methods described herein, for example, wires produced from a non-noble metal can be manufactured to form a biocompatible, for example implantable, platinum electrode.

By means of the methods described herein, implantable or non-implantable medical electrodes and electrochemical sensors, for example, can be produced. Metabolites, for example, such as blood sugar, can be measured using electrochemical sensors. For example, the methods according to the invention can be used to produce medical devices which can be carried in the human or animal body or on the body surface in order to monitor a wide variety of medical parameters. Similar products are also offered for sports and recreational purposes. These can also be produced using the methods described herein.

In some embodiments, the substrate comprises a base metal. In some embodiments, the substrate itself (apart from the noble metal layer according to the invention or prior to the coating therewith) is free of base metals or free of non-metals.

The noble metal ink according to the invention comprises a noble metal. Examples of noble metals according to the invention which can be used for the noble metal ink according to the invention are platinum, iridium, palladium, gold, silver, ruthenium and rhodium. A particularly preferred noble metal for use in the noble metal ink according to the invention is platinum. A further particularly preferred noble metal for use in the noble metal ink according to the invention is gold. In some embodiments, noble metal ink comprises silver.

Particle-free noble metal inks can be produced, for example, using organic noble metal complex compounds. An organic noble metal complex compound suitable for this purpose can, for example, comprise a platinum complex of the type [L1L2Pt[O(CO)R1]X]_(n),

-   wherein L1 and L2 denote the same or different monoolefin ligands or     together denote a compound L1L2 acting as a diolefin ligand, -   wherein X is selected from bromide, chloride, iodide, and -O(CO)R2, -   wherein -O(CO)R1 and -O(CO)R2 denote identical or different     non-aromatic C6-C18 monocarboxylic acid groups, or together denote a     non-aromatic C8-C18 dicarboxylic acid group -O(CO)R1R2(CO)O-, -   wherein said platinum complexes are mononuclear platinum complexes     where n = 1, or wherein, in the event of the presence of L1L2 and/or     -O(CO)R1R2(CO)O-, said platinum complexes may be polynuclear     platinum complexes where a whole number n > 1.

The organic noble metal complex compound can comprise noble metal complexes of platinum, palladium, of rhodium or of iridium, in each case having diolefin and C6-C18 monocarboxylate ligands. More specifically, noble metal complexes of the type [LPd[O(CO)Rl]X]_(n) or [LM[O(CO)R1]]_(n) are provided, wherein L denotes a compound acting as a diolefin ligand, wherein M is selected from rhodium and iridium, wherein X is selected from bromide, chloride, iodide and -O(CO)R2, wherein -O(CO)R1 and -O(CO)R2 denote identical or different non-aromatic C6-C18 monocarboxylic acid groups, wherein the noble metal complexes are mononuclear noble metal complexes where n = 1 or mononuclear noble metal complexes where a whole number n > 1.

The term “compound acting as a diolefin ligand” used herein refers to a compound which, in the noble metal complexes described herein, provides both of, or two of, its olefinic double bonds with a central noble metal atom to form a complex or with two central noble metal atoms in a bridging manner to form a complex.

In the case of polynuclear noble metal complexes described herein, the number n generally denotes a whole number, for example in the range from 2 to 5. In other words, a whole number n > 1 is generally in the range from 2 to 5; in particular, n is in this case equal to 2 and the noble metal complexes are dinuclear noble metal complexes. In particular, the compound L acts as a bridging ligand in the polynuclear noble metal complexes described herein. X can also have a bridging effect.

In the embodiment of mononuclear palladium complexes described herein of the type [LPd[O(CO)R1]X]_(n), L is a compound acting as a diolefin ligand at the central palladium atom; X denotes bromide, chloride, iodide, or -O(CO)R2; and -O(CO)R1 and -O(CO)R2 denote identical or different non-aromatic C6-C18 monocarboxylic acid groups.

In the embodiment of mononuclear noble metal complexes described herein of the type [LM[O(CO)R1]]_(n), L is a compound acting as a diolefin ligand at the central noble metal atom; M denotes rhodium or iridium; and -O(CO)R1 denotes a non-aromatic C6-C18 monocarboxylic acid group.

In a preferred embodiment of dinuclear or polynuclear noble metal complexes described herein of the type [LPd[O(CO)R1]X]_(n), L denotes a compound acting as a diolefin ligand bridging various palladium centers; X denotes bromide, chloride, iodide or -O(CO)R2; n denotes 2, 3, 4 or 5, preferably 2; and -O(CO)R1 and -O(CO)R2 denote identical or different non-aromatic C6-C18 monocarboxylic acid groups.

In a preferred embodiment of dinuclear or polynuclear noble metal complexes described herein of the type [LM[O(CO)R1]]_(n), L denotes a compound acting as a diolefin ligand bridging various noble metal centers; M denotes rhodium or iridium; n denotes 2, 3, 4, or 5, preferably 2; and -O(CO)R1 denotes a non-aromatic C6-C18 monocarboxylic acid group.

Examples of diolefins or compounds of the type L that are capable of acting as diolefin ligands include hydrocarbons, such as COD (1,5-cyclooctadiene), NBD (norbornadiene), COT (cyclooctatetraene), and 1,5-hexadiene, in particular COD and NBD. These are preferably pure hydrocarbons; however, the presence of heteroatoms, for example also in the form of functional groups, is also possible.

X can denote bromide, chloride, iodide, or -O(CO)R2; it preferably denotes chloride or -O(CO)R2, in particular -O(CO)R2.

The non-aromatic monocarboxylic acid groups -O(CO)R1 and -O(CO)R2 in each case denote identical or different non-aromatic C6-C18 monocarboxylic acid groups. The term “non-aromatic” used in this context excludes purely aromatic monocarboxylic acid groups but not araliphatic monocarboxylic acid groups of which the carboxyl function(s) is/are bound to aliphatic carbon. Preferably, -O(CO)R1 and -O(CO)R2 denote identical non-aromatic C6-C18 monocarboxylic acid groups. Among the non-aromatic C6-C18 monocarboxylic acid groups, monocarboxylic acid groups having 8 to 18 carbon atoms, i.e., non-aromatic C8-C18 monocarboxylic acid groups, are preferred.

Examples of non-aromatic C6-C18 or the preferred C8-C18 monocarboxylic acids having the groups -O(CO)R1 or -O(CO)R2 include hexanoic acids, heptanoic acids, octanoic acids, nonanoic acids, and decanoic acids, to name but a few examples. Not only linear representatives but also those having branches and/or cyclic structures, such as 2-ethylhexanoic acid, cyclohexanecarboxylic acid, and neodecanoic acid, are included. The respective groups R1 and R2 bonded to a carboxyl group comprise 5 to 17 or even preferably 7 to 17 carbon atoms.

Preferred examples of platinum complexes comprise [(COD)Pt[O(CO)R1]₂]_(n), wherein n is 1 or 2, and wherein R1 represents a non-aromatic C7-C17 hydrocarbon group, in some embodiments with the exception of a benzyl group.

Preferred examples of palladium complexes described herein include [(COD)Pd[O(CO)R1]₂]_(n) and [(NBD)Pd[O(CO)R1]₂]_(n), wherein n is 1 or 2 and in particular 1, and wherein R1 is a non-aromatic C5-C17 hydrocarbon group.

Preferred examples of rhodium complexes described herein include [(COD)Rh[O(CO)R1]]_(n) and [(NBD)Rh[O(CO)R1]]_(n), wherein n is 1 or 2 and in particular 1, and wherein R1 is a non-aromatic C5-C17 hydrocarbon group.

Preferred examples of iridium complexes described herein include [(COD)Ir[O(CO)R1]]_(n) and [(NBD)Ir[O(CO)R1]]_(n), wherein n is 1 or 2 and in particular 1, and wherein R1 represents a non-aromatic C5-C17 hydrocarbon group.

In some embodiments of the noble metal complexes mentioned herein, a non-aromatic monocarboxylic acid group does not include a phenylacetic acid group.

In some embodiments of the noble metal complexes mentioned herein, R1 does not include benzyl. In some embodiments of the noble metal complexes mentioned herein, R2 does not include benzyl.

The noble metal complexes described herein can be easily produced by ligand exchange, in particular without using carboxylic acid silver salts in the process. The production method comprises mixing or suspending or emulsifying a two-phase system. One phase here comprises a reactant of the type [LPtX₂]_(n), [LPdX₂]_(n) or respectively [LRhX]_(n) or [LIrX]_(n), in each case with X selected from bromide, chloride, and iodide, preferably chloride, either as is or preferably in the form of an at least substantially water-immiscible organic solution of such a reactant. Examples of organic solvents that are suitable for producing such an organic solution and at least substantially water-immiscible also include oxygen-containing solvents, for example corresponding water-immiscible ketones, esters, and ethers, in addition to aromatics and chlorinated hydrocarbons, such as toluene, xylene, di-, tri-, and tetrachloromethane. In contrast, the other phase comprises an aqueous solution of alkali metal salt (in particular sodium or potassium salt) and/or magnesium salt of a C6-C18 monocarboxylic acid of the type R1COOH and optionally additionally of the type R2COOH. The selection of the type of monocarboxylic acid salt(s) depends on the type of noble metal complex described herein that is to be produced or the combination of noble metal complexes described herein that is to be produced. The two phases are intensively mixed, for example by shaking and/or stirring, thereby forming a suspension or an emulsion. Mixing is performed for the purpose of maintaining the suspension or emulsion state, for example for a duration of 0.5 to 24 hours, for example at a temperature in the range from 20 to50° C. The ligand exchange takes place in the process, wherein the formed noble metal complex or complexes described herein dissolve in the organic phase, while the alkali metal X salt or MgX₂ salt that is likewise formed dissolves in the aqueous phase. Upon completion of the suspension or emulsification, organic and aqueous phase are separated from one another. The formed noble metal complex or complexes described herein can be obtained from the organic phase and, optionally, subsequently purified by means of conventional methods.

Examples of preferred noble metal complexes which can be used in connection with the present invention are the platinum complex compounds (COD)Pt[O(CO)CH(C₂H₅)C₄H₉]₂ (also referred to herein as a “PtE complex”), (COD)Pt[O(CO)C(CH₃)₂C₆H₁₃]₂ (also referred to herein as a “PtV complex”), and mixtures thereof.

Examples of gold-containing noble metal compounds which can be used to produce a noble metal ink are disclosed in CN108359316A. Examples of gold-containing noble metal compounds which can be used to produce a noble metal ink are disclosed in EP3597707A1.

In some embodiments, a further noble metal layer can be applied to the noble metal layer, in particular by means of a noble metal ink described herein or another noble metal ink. The further noble metal layer can have structural properties which are the same as or different from the underlying first noble metal layer.

For example, the further noble metal layer can likewise be smooth and completely closed, or the further noble metal layer can be porous.

In some embodiments, appropriate selection of the noble metal complex can influence the morphology of a noble metal layer formed therefrom.

A further aspect relates to a substrate having a noble metal layer which can be produced with the aid of a method described herein. These can in particular be medical devices such as, for example, implantable stimulation or measuring electrodes, electrical or electrochemical sensors, or surgical instruments such as, for example, catheters, stents and stylets. Further implantable medical devices in which a closed outer noble metal layer is advantageous are also provided herein.

A further aspect relates to a noble metal ink comprising less than 10 percent by weight of noble metal, preferably less than 9, 8, 7, 6 or less than 5 percent by weight of noble metal, based on the total weight of the noble metal ink, and a solvent, wherein the noble metal is present as an organic noble metal complex dissolved in the solvent. Such a noble metal ink can preferably be used in the methods described herein.

In such a noble metal ink, the noble metal complexes can preferably comprise diolefin and C6-C18 monocarboxylate ligands of the type [(L1L2)Pt[O(CO)R¹]₂]_(n), [LPd[O(CO)R1]X]_(n), [LRh[O(CO)R1]]_(m) or [LIr[O(CO)R1]]_(m), wherein L denotes a compound acting as a diolefin ligand, wherein X is selected from bromide, chloride, iodide and -O(CO)R2, wherein -O(CO)R1 and -O(CO)R2 denote identical or different non-aromatic C6-C18 monocarboxylic acid groups, and wherein n is a whole number ≥ 1 and m is a whole number ≥ 2.

In a further embodiment of the noble metal ink, the noble metal complex is a compound with the formula [(L1L2)Pt[O(CO)R¹]₂]_(n), wherein n is equal to 1 or 2, L1L2 is cyclooctadiene or norbornadiene, and wherein R¹ represents a non-aromatic C7-C17 hydrocarbon group.

In a further embodiment of the noble metal ink, the noble metal complex is a compound with the formula (COD)Pt[O(CO)C(CH₃)₂C₆H₁₃]₂, which is also referred to herein as a PtV complex.

In a further embodiment, the noble metal ink comprises a solvent comprising propylene glycol n-propyl ether and/or ethanol.

The noble metal ink may further comprise any of the additives mentioned herein.

EXAMPLES

The invention is further illustrated below using examples which are, however, not to be understood as limiting. It will be apparent to a person skilled in the art that other equivalent means may be similarly used in place of the features described here.

Example 1 (Production of (COD)Pd[O(CO)CH(C₂H₅)C₄H₉]₂)

A solution of 35 mmol (COD)PdCl₂ in 200 ml dichloromethane was stirred, and a solution of 140 mmol sodium 2-ethylhexanoate in 150 ml water was added. The two-phase mixture was emulsified for 24 h at 20° C. by vigorous stirring. The dichloromethane phase turned yellow in the process.

The dichloromethane phase was separated, and the solvent was distilled off. The viscous, yellow residue was absorbed into petroleum benzine (40-60), and the solution was dried with magnesium sulfate and filtered. The petroleum benzine was then completely distilled off. A viscous yellow residue of (COD)Pd[O(CO)CH(C₂H₅)C₄H₉]₂ remained. After 10 minutes of heating to 200° C., a specular, 0.5 µm thin layer of palladium could be obtained from a 20 µm thick layer of the (COD)Pd[O(CO)CH(C₂H₅)C₄H₉]₂.

Example 2 (Production of (NBD)Pd[O(CO)CH(C₂H₅)C₄H₉]₂)

Analogously to example 1, 35 mmol (NBD)PdCl₂ in 200 ml dichloromethane were reacted with 140 mmol sodium 2-ethylhexanoate in 150 ml water.

Example 3 (Production of (COD)Rh[O(CO)CH(C₂H₅)C₄H₉])

Analogously to example 1, 16.3 mmol (COD)RhCl in 200 ml dichloromethane were reacted with 65.3 mmol sodium 2-ethylhexanoate in 100 ml water.

Example 4 (Production of (COD)Ir[O(CO)CH(C₂H₅)C₄H₉])

Analogously to example 1, 16.3 mmol (COD)IrCl in 200 ml dichloromethane were reacted with 65.3 mmol sodium 2-ethylhexanoate in 100 ml water.

Example 5 (Production of (COD)Pd[O(CO)(CH₂)_(s)C(CH₃)₃]₂)

Analogously to example 1, 35 mmol (COD)PdCl₂ in 200 ml dichloromethane were reacted with 140 mmol sodium neodecanoate in 150 ml water.

Example 6 (Production of (NBD)Rh[O(CO)CH(C₂H₅)C₄H₉])

Analogously to example 1, 16.3 mmol (NBD)RhCl in 200 ml dichloromethane were reacted with 65.3 mmol sodium 2-ethylhexanoate in 100 ml water.

Example 7 (Synthesis of Pte Complex)

A solution of 65 mmol of (COD)PtCl2 in 100 ml of dichloromethane was stirred, and a solution of 260 mmol of sodium 2-ethylhexanoate in 500 ml of water was added. The two-phase mixture was emulsified for 24 h at 20° C. by vigorous stirring. The dichloromethane phase turned yellow in the process. The dichloromethane phase was separated, and the solvent was distilled off. The viscous, yellow residue was absorbed into 150 ml petroleum benzine (40-60), and the solution was dried with magnesium sulfate and filtered. The petroleum benzine was then completely distilled off. A viscous yellow residue of (COD)Pt[O(CO)CH(C₂H₅)C₄H₉]₂, also referred to herein as “PtE,” remained.

To produce a noble metal ink according to the invention containing the PtE complex dissolved in an organic solvent, the residue obtained above was dissolved in the desired concentration in propylene glycol n-propyl ether (DOWANOL™ PnP Glycol Ether, Dow) in such an amount that the noble metal ink contained the desired amount of platinum, for example five percent by weight, based on the metal content of the noble metal complex in relation to the total weight of the noble metal ink.

Example 8 (Synthesis of Ptv Complex)

A solution of 65 mmol (COD)PtCl2 in 100 ml dichloromethane was stirred, and a solution of 260 mmol sodium neodecanoate (VersaticTM Acid 10, Hexion Inc., Ohio, USA) in 500 ml water was added. The two-phase mixture was emulsified for 24 h at 20° C. by vigorous stirring. The dichloromethane phase turned yellow in the process. The dichloromethane phase was separated, and the solvent was distilled off. The viscous, yellow residue was absorbed into 150 ml petroleum benzine (40-60), and the solution was dried with magnesium sulfate and filtered. The petroleum benzine was then completely distilled off. A viscous yellow residue of (COD)Pt[O(CO)C(CH₃)₂C₆H₁₃]₂, also referred to herein as “PtV,” remained.

To produce a noble metal ink containing the PtV complex dissolved in an organic solvent, the residue obtained above was dissolved in the desired concentration in propylene glycol n-propyl ether in such an amount that the noble metal ink contained the desired amount of platinum, for example five percent by weight, based on the metal content in relation to the total weight of the noble metal ink.

Example 9 (Coating by Means of Dip Coating)

Stainless steel sheets were coated with a noble metal ink according to example 8 by means of dip coating at a constant speed of 0.5 mm/s (ND-R Rotary Dip Coater, Nadatech Innovations). Ink A contained 10 % platinum and propylene glycol n-propyl ether as the solvent, and ink B contained 5 % platinum and propylene glycol n-propyl ether (all figures in percent by weight). Sample A1 was coated once with ink A, sample B3 was coated three times successively with ink B, wherein the ink B was heated between the dip coating steps in each case at 200° C. for 5 minutes in a laboratory furnace (Binder E28) and was thereby decomposed to form a platinum layer. Sample A was subjected to the same curing operation. In both cases, a noble metal layer of pure platinum with a thickness in each case of approximately 50 nm, corresponding to approximately 110 µg of platinum per square centimeter of substrate surface, was formed. This thickness was calculated in each case from the amount of platinum ink used.

In the same way, a wire made of stainless steel was also coated with ink B.

Example 10 (Electron Microscopy)

According to the procedure from example 9, stainless steel sheets were coated three times with ink B (comprising 5 % platinum; sample C3) or once with ink A (comprising 10 % platinum; sample A1). The samples were compared in the electron microscope after the formation of the noble metal layer. Sample C3 showed a significantly smoother morphology than sample A1. Sample A1 had a significantly rougher structure having numerous cracks in the nanometer range. However, the noble metal layer of sample C3 was completely closed.

The thickness of the noble metal layers indicated in example 7 was confirmed using EDX measurements. For this purpose, the ratio of the EDX peaks of platinum and chromium was formed, and compared between samples A1 and C3. The stainless steel sheets of the samples contained a chromium-containing steel. Sample A1 and sample C3 had the same ratio of the EDX peaks of platinum and chromium.

Example 11 (Electrochemical Characterization)

According to example 9, nitinol sheets were each coated three times with ink A (sample A3) or three times with ink B (sample C3) and the ink was decomposed by heating according to example 9 in each case to form a platinum layer. The two samples were measured by means of cyclic voltammetry (Reference 600, Gamry) in phosphate-buffered saline (PBS). In the case of sample A3, a distinct oxidation peak was found here, which could be assigned to the substrate. By contrast, no oxidation peak could be observed in sample C3. Accordingly, such a hermetically closed platinum layer had only formed in the case of sample C3 such that the oxidation of the underlying substrate was prevented.

FIGURES

FIG. 1 shows an embodiment of a method according to the invention for coating a substrate with a noble metal layer. In a first step i), a substrate 101 is provided. In a second step ii), a noble metal ink 103 is applied to the substrate 101 so that a surface of the substrate 101 is covered by the noble metal ink 103. The noble metal ink 103 contains a noble metal, for example in the form of a noble metal complex, and a solvent. In a third step iii), the noble metal ink 103 is heated so that a noble metal layer 102 is formed on the substrate 101. As a result of the heating, all components of the noble metal ink 103 except for the noble metal contained therein are removed from the substrate 102 so that a homogeneous, closed noble metal layer forms on the substrate 101 from the noble metal contained in the noble metal ink 103. For example, the heating can lead to evaporation of the solvent of the noble metal ink and to decomposition of all organic compounds contained in the noble metal ink, which can be dissipated, for example, in the form of gaseous reaction products. These organic compounds can also be noble metal complex compounds, wherein the thermal decomposition removes all organic contents from the ink except for the noble metal.

FIG. 2 shows a wire made of stainless steel which was provided with a layer of platinum according to example 9. The layer formed has no visible irregularities on the micrometer scale and is completely closed.

FIG. 3 shows the surface of a stainless steel sheet which, in accordance with example 10, was provided with a platinum layer with the aid of a noble metal ink having a platinum content of 10 wt.%. The surface has clearly visible cracks in the nanometer range.

FIG. 4 shows the surface of a stainless steel sheet which, in accordance with example 10, was provided with a platinum layer with the aid of a noble metal ink having a platinum content of 5 wt.%. The surface has no visible cracks in the nanometer range. According to EDX analysis, the platinum layer has the same thickness as that shown in FIG. 3 .

FIG. 5 shows a sample as in FIG. 3 , wherein the platinum layer is partially broken open. It is clearly visible that the cracks are located exclusively in the platinum layer and not in the underlying substrate layer.

FIG. 6 shows the results of a cyclic voltammetry measurement of a nitinol sheet which, in accordance with example 9, was provided with a platinum layer with the aid of a noble metal ink having a platinum content of 10 wt.%. The arrow indicates a clearly visible oxidation peak, which is a sign that the underlying substrate is not completely covered by the platinum layer, so that corrosion occurs.

FIG. 7 shows the results of a cyclic voltammetry measurement of a nitinol sheet which, in accordance with example 9, was provided with a platinum layer with the aid of a noble metal ink having a platinum content of 5 wt.%. In this case, no oxidation peak is visible (arrow). 

What is claimed is:
 1. A method for coating a substrate with a noble metal layer, comprising the steps of: (i) providing a substrate, (ii) applying a liquid noble metal ink to the substrate, wherein the noble metal ink contains less than 10 percent by weight of noble metal, preferably less than 9, 8, 7, 6 or less than 5 percent by weight of noble metal, based on the total weight of the noble metal ink, (iii) heating the liquid noble metal ink and thereby forming a noble metal layer on the substrate.
 2. The method according to claim 1, wherein in step (iii) a noble metal layer is formed which has a layer thickness of less than 100 nm, preferably less than 50 nm or less than 30 nm.
 3. The method according to claim 1, wherein in step (iii) a noble metal layer is formed which is completely closed.
 4. The method according to claim 1, wherein in step (iii) a noble metal layer is formed which has a roughness Ra of less than 100 nm.
 5. The method according to claim 1, wherein the noble metal ink is particle-free.
 6. The method according to claim 1, wherein in step (iii) 50 to 500 µg, preferably 50 to 200 µg, of noble metal per cm² of substrate surface are applied to the substrate.
 7. The method according to claim 1, wherein the noble metal layer obtained in step (iii) makes up less than 10 percent by volume, preferably less than 5 or less than 1 percent by volume, of the entire substrate-noble metal layer composite.
 8. The method according to claim 1, wherein the liquid noble metal ink is applied by means of inkjet printing, screen printing, stamp printing, application by means of felt, dispensing, dip coating, spray coating, die coating or spin coating.
 9. The method according to claim 1, wherein the substrate provided with the noble metal layer is suitable for implantation in the animal or human body.
 10. The method according to claim 1, wherein the substrate provided with the noble metal layer is a belt, a cylinder, a tube, a sphere, a wire, or preferably a catheter, a stent, an electrode, a sensor or a stylet.
 11. The method according to claim 1, wherein the substrate provided with the noble metal layer comprises a noble metal, a base metal or a non-metal.
 12. The method according to claim 1, wherein the noble metal ink comprises platinum, silver or gold.
 13. The method according to claim 1, wherein the noble metal ink comprises an organic noble metal complex having diolefin and C6-C18 monocarboxylate ligands of the type [(L1L2)Pt[O(CO)R¹]₂]_(n), [LPd[O(CO)R1]X]_(n), [LRh[O(CO)Rl]]_(m) or [LIr[O(CO)R1]]_(m), wherein L denotes a compound acting as a diolefin ligand, wherein X is selected from bromide, chloride, iodide and -O(CO)R2, wherein -O(CO)R1 and -O(CO)R2 denote identical or different non-aromatic C6-C18 monocarboxylic acid groups, and wherein n is a whole number ≥ 1 and m is a whole number ≥
 2. 14. The method according to claim 13, wherein the organic noble metal complex is a compound with the formula [(L1L2)Pt[O(CO)R¹]₂]_(n), wherein n is equal to 1 or 2, L1L2 is cyclooctadiene or norbornadiene, and wherein R¹ represents a non-aromatic C7-C17 hydrocarbon group.
 15. The method according to claim 13, wherein the noble metal complex is a compound with the formula (COD)Pt[O(CO)C(CH₃)₂C₆H₁₃]₂.
 16. The method according to claim 12, wherein the noble metal ink comprises a solvent which preferably comprises propylene glycol n-propyl ether and/or ethanol.
 17. A substrate having a noble metal layer obtainable by a method according to claim
 1. 18. A noble metal ink comprising less than 10 percent by weight of noble metal, preferably less than 9, 8, 7, 6 or less than 5 percent by weight of noble metal, based on the total weight of the noble metal ink, and a solvent, wherein the noble metal is present as an organic noble metal complex dissolved in the solvent.
 19. The noble metal ink according to claim 18, wherein the noble metal complex comprises diolefin and C6-C18 monocarboxylate ligands of the type [(L1L2)Pt[O(CO)R¹]₂]_(n), [LPd[O(CO)R1]X]_(n), [LRh[O(CO)R1]]_(m) or [LIr[O(CO)R1]]_(m), wherein L denotes a compound acting as a diolefin ligand, wherein X is selected from bromide, chloride, iodide and -O(CO)R2, wherein -O(CO)R1 and -O(CO)R2 denote identical or different non-aromatic C6-C18 monocarboxylic acid groups, and wherein n is a whole number ≥ 1 and m is a whole number ≥
 2. 20. The noble metal ink according to claim 18, wherein the noble metal complex is a compound with the formula [(L1L2)Pt[O(CO)R¹]₂]_(n), wherein n is equal to 1 or 2, L1L2 is cyclooctadiene or norbornadiene, and wherein R¹ represents a non-aromatic C7-C17 hydrocarbon group.
 21. The noble metal ink according to claim 18, wherein the noble metal complex is a compound with the formula (COD)Pt[O(CO)C(CH₃)₂C₆H₁₃]₂. 